1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for displaying a B-mode image or two-dimensional image of blood-flow velocity, using ultrasonic waves and, more particularly, to an ultrasonic imaging apparatus which, with the aid of its visual image, is well suited for examining the heart of a patient.
2. Description of the Related Art
An ultrasonic diagnosis apparatus for forming a B-mode image by means of the ultrasonic imaging technique makes use of an array ultrasonic transducer in which many ultrasonic transducer elements are arranged in a line. When linear scanning is performed by means of the array ultrasonic transducer, a fixed number of ultrasonic transducer elements are driven as a group. To focus an ultrasonic beam, in this case, the ultrasonic transducer elements forming the group are driven at different times; more precisely the transducers located at either end of the group are driven first, with the center transducer being the last to be driven. By driving the array transducer for each of transducer-element groups which are shifted in position one element by one element in the array direction, i.e., the scanning direction, ultrasonic beams corresponding to the respective transducer-element groups or scanning lines can be transmitted in sequence from the array ultrasonic transducer, so that a subject under examination is scanned by the ultrasonic beams.
Echo beams from the subject under examination are received by the same array ultrasonic transducer to be converted into echo signals. The echo signals are subjected to the same delay processing as that to which the transmitted ultrasonic beams were subjected, and are then converted Into a tomograph image signal. The tomography image signal is supplied to a TV monitor device, and a tomograph image is visually displayed.
When, on the other hand, sector scanning is performed, the ultrasonic beams are sequentially transmitted from the ultrasonic transducer in such a way that the subject under examination is scanned in sector fashion, with a given point being designated as the center. In this case, the ultrasonic transducer elements are driven at different times, according to their scanning direction, so that the steering direction of the ultrasonic beam is changed for each scanning line.
In addition to the linear scanning and the sector scanning system as described above, there is a mechanical scanning system in which an ultrasonic transducer is mounted on a mechanical scanning device and is moved thereby to perform ultrasonic scanning.
Ultrasonic echo signals of a subject under examination, obtained by ultrasonic scanning carried out in accordance with the above scanning systems, are processed to produce an image signal which, in general, is visually displayed as a B-mode image (tomograph image).
The blood-flow imaging technique for displaying a blood flow profile has been made feasible owing to the ultrasonic imaging apparatus. The Doppler method is generally used for the blood flow imaging. This is a method for detecting moving substances, such as blood flow, within a living subject utilizing the Doppler effect.
The basic principle of the Doppler method is as follows:
When the blood flow within a living subject is subjected to ultrasonic beams, blood corpuscles are caused to vibrate slightly while moving, and reflect the ultrasonic beams. Thus, the reflected beams are subjected to the Doppler effect. In this case, when the blood corpuscles are moving toward the ultrasonic beams, the frequency of the reflected beams becomes slightly higher than that of the transmitted beams, while when they are moving away therefrom, it becomes slightly lower than that of the transmitted beams. Such a frequency shift can be detected, the amount of change in the frequency being referred to as the Doppler shift frequency. Since the Doppler shift frequency is in proportion to the blood-flow velocity, this enables the blood flow conditions to be then be clearly observed.
According to a conventional blood-flow imaging apparatus working on the above principle to obtain blood-flow information, a predetermined number of ultrasonic pulses are repeatedly transmitted in a given direction, and the resulting echo waves are converted in sequence, into echo signals. The echo signals are then phase detected to obtain phase information signals which are in turn digitized and supplied to a digital filter, which removes therefrom signal components corresponding to entirely or virtually motionless parts within the living subject. The signals passed through the digital filter are frequency-analyzed to detect the Doppler shift frequency corresponding to the blood-flow velocity. The Doppler shift frequency is used as blood-flow information for forming a two-dimensional image or profile of the blood-flow velocity. The blood-flow information may be displayed on a TV monitor, either independently or superposed on B-mode or M-mode information.
To simultaneously display a two-dimensional image of blood-flow velocity, superposed on the B-mode or M-mode image, a Doppler information detecting transducer is provided, separated from or integrated with a linear-or sector-scanning array ultrasonic transducer. For instance, ultrasonic waves are transmitted to a subject in accordance with the sector scan to obtain B-mode information, and ultrasonic pulses are transmitted to the subject to obtain Doppler information. The thus obtained B-mode information and Doppler information are superposed and displayed on the monitor TV.
To obtain high-accuracy blood-flow information, that is, Doppler information, each scan line is scanned plural times, for example, ten times, and a sufficient time corresponding to ten scans is taken to obtain the Doppler information at each sampling point. For this reason, a considerable time of about 100 msec is needed to construct a frame of image. This will result in a significant time difference in a two-dimensional image obtained during this period of time. That is, a significant time difference occurs between the first scan region and the last scan region. Accordingly, even if an image of relatively rapid blood-flow, such as the blood-flow in the heart, is displayed on the entire screen of a display, this would not mean, because of the above time difference, that the blood-flow image were precisely displayed. This will make a correct diagnosis difficult.